Evaluating future colonizable habitats of four oak species in the eastern United States using decision-tree based ensemble and cellular simulation models

Background/Question/Methods

Oak species have experienced a decline in the eastern U.S. resulting in changes in forest composition. The main mechanisms for this decline are altered fire regimes resulting in changes in canopy structure and competitive ability, and susceptibility to insects. However, our decision-tree based ensemble model (DISTRIB) predicts a large increase in potential oak habitats (http://www.nrs.fs.fed.us/atlas/tree/fut_fortypes.html) under future climatic conditions (Hadley CM3 GCM model under high carbon emission scenario). This results in expansion of oak habitats in the north and northeastward direction by the year 2100, driven mainly by Quercus alba, Quercus prinus, Quercus stellata and Quercus velutina.

Increases in habitat does not necessarily mean colonization of these sites under current fragmented landscapes. So, we investigated the colonization potential of these four tree species using a spatially explicit cell-based simulation model (SHIFT). SHIFT calculates the probability of an unoccupied cell becoming colonized in each generation as a function of habitat quality and abundance of the species by implementing an inverse distance kernel for the search-distance function under several estimates of historical migration rates. We also evaluated the colonization probabilities of these species under future climate by intersecting the output of DISTRIB modelled future habitats with the colonization probabilities calculated by SHIFT. In addition to modelled outputs, we considered other disturbance and biological factors that can change the outcome for these four oaks in a decision support framework.

Results/Conclusions

Although future habitat increases of oaks (especially Quercus stellata) results in a large increase of the oak-hickory forest type (with the hickories being a minor contributor to the increase) according to DISTRIB, SHIFT shows that the advancing front of the colonizing species will be concentrated in the first 10-20 km from the current boundary (although our model allows for rare long distance migration of up to 500 km from the current boundary), greatly limiting the actual colonization potential of these habitats by the year 2100. We establish that source strength, and to a lesser extent, sink strength, is critical in driving colonization at the range margins. Also, factors such as uncertain fire intervals in future anthropogenic landscapes and the increased incidence of disease and insect attack because of warming climate means that oak species will continue to be under great stress and may fail to establish in suitable habitats. We are currently planning to explore sub-models that could shed light on finer abiotic and biotic aspects of dispersal at the boundary.